The invention relates to new and useful improvements in front systems for electrical printed circuit boards. More particularly, the invention relates to improvements in arrangements used for levering such front systems into and out of mounting racks.
U.S. Pat. No. 4,233,646 discloses a lever pull handle for printed circuit boards. There is an engagement region fashioned as an open-ended wrench in the region of a lower fulcrum. When the lever is operated, a printed circuit board can be levered into or out of a mounting rack via this engagement region. Above the fulcrum, the handle has an elongated lever region, which ends in a handle piece. There is a region in the form of a hook at the junction between the lever region and the handle piece. This allows the lever to be locked on a pin which is arranged on the printed circuit board, so that the lever pull handle rests approximately parallel to and against the front edge of the printed circuit board.
Such an arrangement has the following disadvantage. If, for example, the printed circuit board is in a state where it is completely inserted into the mounting rack and if, for example, the intention is to lever it out of the mounting rack, then an operator must initially exert force to move the lever out of the locked position. As a result, the region formed as a hook is separated from the pin, thereby allowing the lever to be pivoted about the fulcrum such that the engagement region can come into contact with the mounting rack. Only then is it possible for an operator to exert further force on the handle piece to lever the assembly out of the mounting rack. In the process, the problem arises that an operator cannot reliably distinguish between the two successive processes of applying force. In fact, there is a risk that the unlocking of the lever pull handle by the first process of applying force will merge directly into the process of levering the entire printed circuit board out of the mounting rack. In practice, this is increasingly undesirable. While the unlocking of the lever pull handle from the locked position represents a simple preparatory action, the process of completely levering a printed circuit board out of a mounting rack is a drastic action when such a printed circuit board is part of an overall electrical system which, in many cases, comprises a large number of printed circuit boards. In order to prepare for this, an operator must in some circumstances carry out other, different measures, for example in the simplest case disconnecting the power supplies. For this reason, an inadvertent, virtually smooth transition between the simple unlocking of a lever pull handle on the front face of a printed circuit board and the actual process of levering the printed circuit board out of the mounting rack is undesirable, for example for safety reasons.
European Patent EP 0 129 883 B1 discloses a further version of a lever pull handle, which corresponds to the type disclosed in U.S. Pat. No. 4,233,646. In this case, a leaf spring projects from the front face of the handle piece of the angled lever. This leaf spring is directed toward the front strip of the printed circuit board assembly, and is fitted at the end thereof with a latching tab. This latching tab can be latched in a window-like opening in the front strip in order to lock the lever. In order to unlock the lever from this locked position, an operator must push the leaf spring in by exerting force on the handle piece, and must at the same time pivot the lever. This can be done only by exerting considerable force. As a result, the problem that has already been described with respect to U.S. Pat. No. 4,233,646 also occurs here, namely the lack of any distinction between the preparatory process of unlocking the lever and the main process of levering the entire printed circuit board out.
European Patent EP 0 194 515 B1 discloses a further version of a lever pull handle which likewise, in principle, corresponds to the design described in U.S. Pat. No. 4,233,616 and EP 0 129 883 B1. Once again, the lever can be locked via a latching tab on a leaf spring in a position which rests as far as possible against the surface of the front panel of a printed circuit board assembly. A spiral spring is mounted in the interior of the lever pull handle. This spiral spring exerts a pre-stress on the entire lever, which is mounted such that it can rotate, such that the lever pivots as far as possible in the direction of the locked position, and/or snaps back on its own toward this position. The disclosures of U.S. Pat. No. 4,233,646, EP 129 883 B1 and EP 0 194 515 B1 are incorporated into he present application by reference.